Notes On Chapter Thirty -- Example Of A Client And A Server

• 30.1 Introduction
  
  
  • In this chapter we see explicit example code that uses the socket API to implement a simple client and a simple server.
    
    
• 30.2 Connection-Oriented Communication
  
  
  • In connection-oriented communication the protocol software establishes the connection.
    
    
  • In order to establish a connection applications at both ends must execute commands directing actions of protocol software.
    
    
• 30.3 An Example Service
  
  
  • The idea of the example service is just to count the number of client requests received and report the current count to each client.
    
    
  • This particular example server is connection-oriented.
    
    
• 30.4 Command-Line Arguments For the Example Programs
  
  
  • The server has an optional port parameter. The default port is 5193.
    
    
  • The client has optional hostname and port parameters. Default host is localhost and default port is 5193.
    
    
  • While debugging, client and server can the same host -- just use the defaults.
    
    
  
• 30.5 Sequence Of Socket Procedure Calls
  
  
  • Study the diagram on page 451 of the fourth edition.
    
    
  • It illustrates the actions of the client and server.
    
    
• 30.6 Code For Example Client
  
  
  • The code for the client starts on page 452 of the fourth edition.
    
    
  • In class, I'll show on screen the comments I have in my copy of the text
    
    
• 30.7 Code For Example Server
  
  
  • The code for the server starts on page 455 of the fourth edition.
    
    
  • In class, I'll show on screen the comments I have in my copy of the text
    
    
• 30.8 Stream Service And Multiple Recv Calls
  
  
  • TCP guarantees only that data will be delivered in order, with each call to recv() returning one or more octets until all data has been received.
    
    
  • This accounts for the manner in which the client reads in a loop waiting for a return value of 0 bytes, even though the server sends all the data across with one call to send()
    
    
• 30.9 Socket Procedures And Blocking
  
  
  • In Unix the calls to the socket operations are system calls.
    
    
    • They are requests for service to the operating system.
      
      
    • It is typical that client and server are suspended (aka blocked) while the operating system performs requested functions.
      
      
    • Most of the socket functions work this way.
      
      
    • It's good that the programs block until the system calls return. There is nothing for them to do, and they use up system resources like CPU time if they are allowed to run while doing no productive work.
      
      
  • The effect of the blocking is to synchronize the client and server.
    
    
  • For example: if the server is slow to get to the part of the program where it does an accept, then the client just "sleeps" waiting to complete the call to connect(). This allows the server to "catch up" with the client.
    
    
• 30.10 Size Of The Code And Error Reporting
  
  
  • The kind of error checking you see in the client and server code is typical of "system programmer code."
    
    
• 30.11 Using The Example Client With Another Service
  
  
  • You can use either client or server with a "wrong" correspondent! Mix and match!
    
    
  • Use DAYTIME service (port 13 on many hosts) to test the client independently of the server.
    
    
• 30.12 Using Another Client To Test The Server
  
  
  • You can use telnet to test the server independently of the client (incidentally we did something like this earlier in lab when we telnetted to the "sendmail" port 25 and delivered "fake" e-mail.
    
    
  • Example: telnet zaurak.csustan.edu 5193
    
    
• 30.13 Summary